1. Field of The Invention
The present invention relates to a method for manufacturing a lead frame or substrate, and in particular, to a method for manufacturing a lead frame or substrate for an electronic device such as a DIP or Flat Pack Package wherein a semiconductor or the like is mounted on the lead frame or substrate or the like in the electronic device and sealed in a molding material.
2. Description of the Prior Art
FIG. 1 shows the cross section of one part of a conventional electronic device. In FIG. 1, a lead frame 1 is made of a metal such as a copper, 42ALLOY or the like, and comprises a die 1a for mounting a semiconductor chip 3, and a lead section, that is an inner lead 1b and an outer lead 1c. The semiconductor chip 3 is die-attached e.g. by printing gold onto the surface of the die 1a, so that a gold-silicon alloy is formed at 300.degree. C. to 400.degree. C., and the semiconductor chip 3 is mounted using the gold-silicon alloy as an adhesive 5. In place of the gold-silicon alloy, it is also possible to use an adhesive such as an organic resin of a polyimide type or of an epoxy type, or a silver paste containing an organic material, or the like. An excess 5a of the adhesive is squeezed out during the mounting operation as shown in the drawing. The semiconductor chip 3 and the inner lead 1b are wire-bonded using a gold wire 9, after which the entire package is sealed in a plastic molding 7 with the exception of the outer lead 1c which is to be exposed.
When an organic adhesive is used as the adhesive 5, the adhesive in the flowable state is hardened by a chemical or thermochemical reaction, so that the semiconductor chip is mounted to the die 1a. This method has the advantages that it is inexpensive and easily adaptable to mass production. However, on the other hand, the adhesive 5 and the excess adhesive 5a are gradually vaporized and moved as indicated by the arrow 5b when heated in the range of 100.degree. to 300.degree. C., and an adhering material 5c is formed on the surface of the semiconductor chip 3 and lead frame 1. In addition, natural oxide and a suboxide 15 are formed on the lead frame 1. The adhering material 5c and the suboxide 15 are extremely easy to strip from the surface of the lead frame and therefore end up damaging the adherence of the subsequently-applied molding and protective film to the lead frame 1.
At the rear surface of the lead frame 1, it is easy for moisture to permeate into the plastic molding 7 from outside and to be accumulated thereon. When the device is usually dipped into molten solder, due to heat distortion or sudden expansion of vaporizing water, stress is produced causing the molding to expand. For this reason, cracks 11 and voids 13 are produced at the boundary between the die 1a and the molding 7 in areas where the adherence is poor.